In the manufacture of an electronic device such as a semiconductor device or the like, an etching process is performed on an etching target layer of a processing target object. A plasma etching may be used for this etching process. The plasma etching may include an isotropic etching in which an etching progresses in all direction; and an anisotropic etching in which an etching progresses only in a certain direction. The isotropic etching or the anisotropic etching may be selectively performed depending on a required etching shape. In the isotropic etching, since the etching progresses to form a circular arc-shaped cross section, it may be possible to etch an etching target layer such that a part of the etching target layer located under a mask pattern is removed, for example.
A processing method of an etching target layer by performing the isotropic etching is described in Patent Document 1. In Patent Document 1, it is described that a part of a buried oxide film, which serves as an etching target layer located under a fin, is removed and undercut in a horizontal direction through a dry etching process, so that a recess is formed.
Patent Document 1: Japanese Patent Laid-open Publication No. 2006-522486
When etching an etching target layer containing silicon, a breakthrough process for removing a natural oxide film formed on a surface of the silicon as a result of being exposed to the atmosphere needs to be performed as a pre-treatment. In this breakthrough process, fluorine radicals are generated by exciting plasma of a processing gas containing fluorocarbon such as CF4, in general, and the natural oxide film on the surface of the silicon is removed by the fluorine radicals. At this time, in order to suppress a carbon-based residue, which is originated from the fluorocarbon, from adhering to a surface of a processing target object, an O2 gas may be added into the processing gas.
If, however, the O2 gas is added into the processing gas in the breakthrough process, the etching target layer may be re-oxidized by the O2 gas, and an oxide-based residue originated from SiO2 may be deposited on the surface of the etching target layer. As a result, in a main etching process performed after the breakthrough process, this oxide-based residue may serve as a micro mask, so that etching uniformity on the surface of the etching target layer is deteriorated. Especially, in the isotropic etching, since the etching is performed by setting a bias voltage to be low to reduce an influence from ions in the plasma, the etching uniformity may be largely affected by a surface state of the processing target object. Thus, when performing the isotropic etching on the processing target object as described in Patent Document 1, the etching uniformity may be degraded due to the oxide-based residue generated in the breakthrough process.
Further, in order to suppress a carbon-based residue and an oxide-based residue from being generated, it may be considered to use Cl2 or HBr as an etchant in the breakthrough process. Since, however, these etchants have low selectivity against silicon, silicon itself may also be etched. As a result, it may be difficult to control a recess shape.
Accordingly, in the technical field, there has been a demand for an etching method capable of implementing the isotropic etching with high uniformity.